Switchable field doesn't keep the defaultValue on multi language componentes

Steps to reproduce

1.  Create a switchable field like the example we provide at our docu
   myComponent.yaml

   form:
     properties:
       switchable:
         $type: switchableField
         i18n: true
         field:
           $type: radioButtonGroupField
           layout: horizontal
           defaultValue: foo
           datasource:
             $type: optionListDatasource
             options:
               - name: foo
                 value: foo
               - name: bar
                 value: bar
         itemProvider:
           $type: jcrChildNodeProvider
         forms:
           - name: foo
             properties:
               foo:
                 $type: textField
           - name: bar
             properties:
               bar:
                 $type: richTextField

2.  Create the component on a site which has multiple languages enabled
3.  Check the defaultValue is selected when opening the dialog
4.  Switch from fallback local to another one
5.  Check the defaultValue is not selected

Expected results

When user switch language, the defaultValue is selected

Actual results

The defaultValue is only selected for the default language when opening the dialog

Workaround

N/A

Development notes

Tested on Magnolia 5.7 and the "selected" option was working as expected. When you switch between languages, the "selected" option is always properly marked.

Anyhow, even using the old UI 5 dialog definition, it doesn't work on Magnolia 6, e.g:

form:
  tabs:
    - name: tabText
      label: Text
      fields:
        - name: message
          class: info.magnolia.ui.form.field.definition.SwitchableFieldDefinition
          label: Message
          i18n: true
          options:
            - name: plainText
              label: Plain
              selected: true
              value: plainText
            - name: richText
              label: Rich
              value: richText
          fields:
            - name: plainText
              class: info.magnolia.ui.form.field.definition.TextFieldDefinition
              label: Plain text message
              rows: 5
            - name: richText
              class: info.magnolia.ui.form.field.definition.RichTextFieldDefinition
              label: Rich text message
              height: 200

actions:
  commit:
    class: info.magnolia.ui.admincentral.dialog.action.SaveDialogActionDefinition
  cancel:
    class: info.magnolia.ui.admincentral.dialog.action.CancelDialogActionDefinition